Television receiver including black-level control



TELEVISION RECEIVER INCLUDING BLACK-LEVEL CONTROL Filed July 27, 1938 FREQUENCY TRANSLATOR 5 4: r SEPARATOR Fna sgncv' i GENERATOR 43 1 5 +Sc.+B 2 12 1 k FIELD FREQUENCY, GENERATOR FIGZ. x

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: Time O O m "PI" :11: YT WT? l W W? ATTORN EY Patented July 15, 1941 UNITED STATES PATENT OF FiI'CiE Harold M. Lewis, Great .Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application July 2-7, 1938, Serial No. 221,497 9 Claims. .(Cl. 1 Z87.3)

This invention relates to television receiving systems and more particularly to arrangements for automatically controlling one or more operating characteristics of such systems.

In accordance with the present televisionpractice, a transmitted signal comprises -a carrier wave modulated during successive intervals or trace periods by high-frequency and unidirectional components representative of variations in light over-successive incrementalareas of an image being transmitted and of its average background illumination, respectively. The carrier wave is unmodulated by light-representative components during retrace periods, ordinarily providing 'a level'during such periods which has a fixed relation to black, but is modulated during such periods by synchronizing pulses which correspond to initiations of successivelines and fields in the scanning-of the image.

a At the receiver a scanning beam isdeveloped and so deflected as to scan andilluminate a target in a series of fields of parallel lines. 'The synchronizing components of the received signal are utilized to control the scamiing apparatus of the receiver so as to synchronize its operation with that of similar apparatus utilized at the transmitter in-developing the signal. The intensity of the beam is controlled by the light modulation components, thereby to reconstruct the image. i

In certain proposed television systems, the video-frequency modulation developed from the image is so impressed on'the carrier'that increases in the carrier amplitude correspond to increases in brightness of the image, this being termed positive modulation. The background illumination is transmitted directly, that is, by variation of the average carrier intensity during the trace intervals directly in accordance with the low frequency or steady background light components. Between the trace periods, a blanking signal or level is,-as stated above, transmitted and corresponds in amplitude to black or a reference level fixed in value with respect to black but blacker than black. The synchronizing pulses are impressed on the carrier during the blanking intervals'in the opposite sense to the light modulation signals, ordinarily periodically reducing the carrier amplitude to zero.

While systems utilizing the type of signal just described have, in general, given satisfactory results, certain difficulties are incident to their use.

For example, an important consideration in providing the proper operation ofatelevision repulse components, together with means for 'ceiver is the stabilization of thedetected signal, whether derived from a positively or-negatively modulated carrier, with respect to its black level. More particularly, a fixed level for the detected signal amplitude corresponding to black must -be established in order that all of such components will "be-represented by black in the reproduced signal and the light gradations-from black to white represented by thevarious video-frequency signal components willbe properly represented in thereproduced scene.

-It is an object, therefore, of the present invention to provide 'a television receiver including improved means -'-for automatically controlling certain operating characteristics of the receiver in accordance with character-istics-of the received modulated-carrier signals. V It is-a further-object of the invention to provide an improved automatic black-levelcontro'l system in conjunction with at least one other system for-adjusting another operating characteristic for television receivers adapted for the reception of positively modulated televisiomsignals including background and'high-frequency light-modulation components by which the amplification and at least one other operating characteristic of a receiver may be controlled.

It is a further object of the invention :to *provide 'a television receiver including improved means -for automatically establishing a fixed sig: nal level for the components corresponding to black in the-detected signal. I It is a further object of the invention to provide a television receiver embodying improved control means foreffecting both automatic-amplification-control and establishment of a fixed signal level for the components corresponding to black in the -detected signal. I

In accordance with the invention, a'television receiver, adapted forthe reception of a television signal-carrier wave the amplitude of which may vary at thereceiver and modulated during trace periods by light components having a predetermined blanking level, unmodulated by light components and modulated by synchronizing-pulse components outside the amplitude range of the light-modulation components during retrace periods, comprises means for deriving from areceived signal its modulation signal stabilized with respect to the peak of the synchronizing- =blocking out only the synchronizing-pulse components from the derived signal. The receiver also includes means for stabilizing the resultant derived signal'with respectto the blanking level,

together with means for utilizing the blankinglevel-stabilized signal to reproduce the transmitted light-modulation components.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the'accompanying drawing, and its scope will be pointed out in the appended claims.

In the accompanying drawing, Fig. l is a circuit diagram, partially schematic, of a television receiving system including circuits embodying the frequency voltages impressed upon its control present invention; while Figs. 2-6, inclusive, are v graphs representing the wave forms of the signals as developed at different points in the receiver of Fig. 1.

Referring more particularly to Fig. l of the drawing, the system there illustrated comprises a television receiver which may be of either the tuned radio-frequency or superheterodyne type and including, in cascade in the order named, an antenna system I l l2, a carrier-frequency translator l3, a detector and video-frequency amplifier indicated generally at M, a further videofrequency amplifier l5, a stabilizer l6, and an image reproducing device ll. Where the receiver isof the superheterodyne type, the conventional frequency changer and intermediate-frequency amplifier are included in the'translator l3.

There is also connected to the output circuit of the translator l3, in cascade in the order named, a detector l8, a first separator l9, and an inter-synchronizing signal separator 20. A linefrequency scanning wave generator 21 and a fieldfrequency scanning wave generator 22 are coupled to the outputcircuit of the separator 28 and are connected to the scanning elements of the device I! in a conventional manner. The parts of the receiver illustrated in detail are con structed in accordance with the present inveng tion and will be hereinafter further described. Those parts of the system represented schematically may be conventional in construction and operation. I

The system of Fig. 1, as described above, thus includes all the conventional components of a television receiver for receiving a television signal the amplitude of which may vary at the receiver. The circuits being, in general, well understood in the art, a detailed description of their gen.- eral operation is unnecessary. Briefly, however, vision-modulated carrier waves are intercepted by the antenna il, l2 and selectively amplified in the carrier-frequency translator [3. Where the receiver is of the superheterodyne type the signals are impressed on a frequency changer in the translator l3 where they are converted into intermediate-frequency signals and thereupon further selectively amplified. The amplified signal from the translator I3 is delivered to the detector and video-frequency amplifier I4 wherein there are derived and amplified the modulationfrequency components, including video-frequency and synchronizing components. The modulation-frequency components are thereupon sup? plied to the video-frequency amplifier l5 where they are further amplified and supplied by way of the stabilizer I B to a brilliancy control element of the device ll. p

. ,The detector l8 also derives the modulation signal from the carrier output of translator l3 andtranslates the same to theseparator l9, wherin the synchronizing-impulse components are separated from the video-frequency components, as further to be explainedlater. The

element in the usual manner. Saw-tooth scanfning waves developed by the generators 2| and 22 under the control of the synchronizing-pulse components are utilized in the conventional manner to produce electric fields of saw-tooth wave form to deflect the beam in two directions normal to each other, thereby to trace the scanning pattern upon the screen of the device I! and to reconstruct the transmitted image.

- Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, the detector and amplifier I4 comprises a vacuum tube 23, of the diode-triode type or equivalent. The diode section 24 is supplied from an inductance element 26 coupled to an inductance element 29 included in the output circuit of the carrier-frequency translator and is provided with a load circuit including a resistor 21 and condenser 28 in parallel, as shown. The negative end of the resistor 21 is directly connected to the control grid of the triode section 25 of the tube 23. The loadcircuit of the triode section 25 comprises a resistor 30 included in its cathode circuit. Operating potential is supplied to the anode of the tube from a suitable source, indi cated at +3. The diode 24 operates in a conventionalmanner toderive the modulation voltages from the carrier signal and impress them upon the triode control grid. The triode section 25 also operates conventionally to amplify these voltages so that the amplified modulation signal voltages are developed across the load resistor 30.

The, amplifier l5 may comprise a conventional pentode tube 3| having its input circuit connected across the resistor 30 as shown, a suitable biasing battery 32 being included in its cathode circuit. A load resistor 33 and choke 34 are included in the anode circuit while operating potentials are applied to the anode and screen from suitable sources, indicated at +B and +80, respectively. a 4 7 The control element of the device H, which may for example, be the input electrode of a cathode-ray tube, is connected across the load circuit 33, 34 of the amplifier l5 by way of a coupling condenser 35. For the purpose of stabilizing the modulation signal with respect to its blanking, level or the components thereof corresponding to black, there is provided the stabilizer I6 comprising a diode 36 and a parallel load resistor.3'|, connected across the input circuit of the cathode-ray tube. A fixed biasing potential maybe applied to the control element of the device IT, for example, the input control electrode of a cathode-ray tube, by way of the resistor 31 from a suitable'source indicated at G.

For the purpose of separately deriving and separating the synchronizing-pulse components from the received signal carrier, there are provided the detector l8 and separator f9. The detector l8 comprises a diode 38 fed from an inductance element 4| coupled to the inductance element 29 in the output circuit of the translator I3 and {provided with a load circuit including a masses resistor 39 and condenser 40 in parallel. The separator-l 9 comprises an amplifier tube preferabl-y'of the pentode type and having itsinput circuit connected across the load circuit 39, 40 and having-abiasing resistor 43 and by-pass condenser 44 included in its cathode circuit and a load resistorfiS included in its anode circuit. Operating potential may be applied to the anode of the tube -42, by way of the resistor 45, and directly to the screen from suitable sources indicated at +Band +Sc, respectively. The input circuit of the separator 2-0 is connected across the load resistor 45.

The tube 42 is so biased and its grid voltageplate current characteristic is such that, with signal input voltages of normal amplitude supplied from the detector [8, it operates beyond cutofi so that only the synchronizing pulses of the detected modulation signal are passedby this amplifier and developed in its output circuit. That is to say, the synchronizing-pulse components are separated from the video-frequency components, the former having a wave form such as indicated adjacent the resistor 45 in Fig. 1. The output circuit of the limiter I9 is also connected by way of a suitable lead 36 including a coupling condenser 47 to thecontrol grid of the triode section 25 of the tube 23 and, by way of a suitable lead 48 including a coupling condenser 49,-to a control element of the device, for example,the input electrode of acathode-ray tube. Thus, the synchronizing-pulse components as developed in the output circuit of the limiter I9 are applied negatively to control electrodes or elements of the separator 20, the amplifier 23 and the device ll.

- For the purpose of developinga unidirectional bias voltage for efiecting-automatic amplification control, there is provided a rectifier indicated generally at 50 comprising a diode having a load circuit including a resistor 52 and condenser 53. The diode and its load circuit are effectively connected across the load resistor 3 of amplifier -14. For the purpose of suitably biasing the. diode 50, which operates as a peak detector, a positive potential is applied to its cathode by way of the resistor 52 from a suitable source, indicated as the battery 54, to compensate for the positive bias on its anode applied from bias resistor 30.

The operation and results obtained by the system of Fig. 1 may best be understood by reference to the curves of Figs. 2-6, inclusive, which illustrate the wave forms of the signal as developed at various points in the system. In Fig. 2, there is illustrated the wave form of a portion of the complete modulation envelope of a television wave of the positively modulated type, such as may be developed in the output circuit of the carrier-frequency translator l3. The portion of the curve of Fig. 2 which is shown at A represents a wave of relatively great amplitude while that at A represents a similar wave different only in that it is of lesser amplitude, as might a tions of successive lines, beingmodulated as-indicated at M1 and M2 by the high-frequency and backgroundlightcomponents except during field block-out periods.

field-blackout intervals ordinarily include the line-synchronizingpulses as well as additional broad pulses, not shown, which latter may constitute collectively a frame-synchronizing pulse. Ordinarily, the blanking levelof the wave indicated-at Xcorresponds to black in-the scene being-transmitted or a-fixed value relative thereto which is blacker than black-and maybe termed 'thebla-ck level.

I "It is well known that, with -the "positive type of-modulation such as is representedin Fig. increases of the light intensity of the image beingscanned are represented by increases in the amplitude '-of the carrier and the average intensity of the signal is-varied in accordance with the low-frequency or unidirectionalbackgroundillumination-components of the'image. Thus, in the particular portions of the wave shown, duringthe line-trace modulation periods-represented at M1, the amplitude of the-carrier is such that it reaches the white level, whereas this does not occur during the modulation periods represented at M2. Since there are no regularly recurring peak values, corresponding to the white level in a positively modulatedsignal, the peak value of the signal cannot be used 'for the purpose of automatic amplification control; Nor, as stated above, are-there inth-is type of wave' any synchronizing pulses which-could be utilized for the purpose in question, since the signal is reduced to zero for the duration of the line-synchroniz- In the operation of -the present invention,

therefore, the modulated-carrier wave is detected by the diode detector 24 of tube 23 to derive a voltage across its load resistor 21 corresponding to half of the modulation envelope, which voltage is impressed upon the triodeamplifier portion 25 of the tube 23. Since the connection between the resistor 21 and the amplifier control electrode is-a direct one, the signal as applied to this amplifier is stabilized with respect to the synchronizing pulses; that is,the peaks of the synchronizing pulses are maintained at a substantially fixed level as impressed on the control grid. The stabilized signal input to amplifier 25 is shown in Fig. 3, the synchronizing pulses being indicated in dotted lines. Were the connection between the diode and the controlgrid not direct, as by way of a coupling condenser, the unidirectional component of the signal would be lost and it would tend to establish itself about its neutral axis so that the levels ofthe peaks of the synchronizing pulses would depend on the average signal amplitude; that is, the-signal would not be stabilized with respect =to-the synchronizing pulses. 7

Apart "of'one of the fieldblockout intervals is indicated at YY. The

The diode I8 and limiter l9 operate, asexplained above, to detect the signal derived from the output circuit of the translator l3 and to limit and amplify the derived modulation signal, thereby to develop a negatively'poled synchronizing-pulseisignal across theload resistor 45 and this signal, as explained above, is applied to the control electrodes of the triode section 25 of tube 23, theseparator gil and cathode-ray tube ll. This pure, synchro nizing -pulse signal is applied to the gridoi amplifier section 2 with a polarity opposite to that of the synchronizingpulse components in the signal derived from the diode detector section 24 so that the original synchronizing pulses are removed from or blocked out of, the; resultant signal and the inserted pulses appear in the signal output of amplifier 25 with opposite polarity from that of the original pulses. This modified signal as developed across the load resistor 3D, therefore, is of a wave form such as illustrated in Fig. 3, theoriginal synchronizing pulses being indicated by the broken lines as they would have appeared if not blocked out, and the inserted pulses with reverse polarity being indicated by Li. It will be noted that the blanking level X still'corresponds to its level in the original signal; that is, the modulation signal is still stabilized with respect to the peaks of the synchronizing pulses. This level, therefore, is a measure of theaverage amplitude of the carrier independent of light modulation and can be used to effect a control of any desired characteristic of the receiver, for example, automatic amplification control. To this end, the diode 5i, the load circuit 52, 53 of which has a time constant which is preferably long as compared to the line-scanning period, detects the peak value in the positive or black direction of the signal developed across the resistor 30 and, hence, develops a control-bias voltage varying in accordance with the peak value of the wave of Fig. 3. Thus, this bias voltage is responsive to a predetermined level of the modulation envelope, in the present case the blanking level, and independent of othercharacteristics thereof; it is entirely independent of pulses which depend upon periodicity of synchronizing of the signal. This control-bias voltage is dependent only on the blanking level X, the nature of the video signal being-immaterial as all components thereof are of lesser amplitude so that the peak detector 5| is not responsive thereto, the detector being effective to peak-rectify the shoulder on each side of the line-frequency pulses of Fig. 3. This shoulder is always presentto represent a black level since the synchronizing pulses are of less duration than the blanking intervals. This control-bias voltage varies inversely in accordance with the carrier amplitude as unaffected by modulation components and is applied positively by way of the lead indicated as A. V. C'. to the control electrodes of one or more of the tubes in the translator l3 which havea large initial negative bias (not shown) The resultant negative bias utilized for controlling the amplification of translator I3 is thus as illustrated by the curve of Fig. 4, the variation in its value for signals of greater and lesser amplitudes being indicated at a. and (11, respectively. Thus the amplification in these tubes is controlled inversely in accordance with variations of the average carrier intensity and independently of light -modulation components. A suitable conventional filter (not shown) may be included in the AVG lead for the purpose of removing residual fluctuations in the bias voltage. Thus, the output signal intensity of the channel of the translator is maintained within a relatively narrow range for a wide range of received signal intensities. I

As explained above, the signal developed in the output circuit of the detector and amplifier I4 is also applied by way of the amplifier l5 and stabilizer It to the cathode-ray tube IT. For the purpose of properly fixing the level of black in the signal as applied to the control element of the device Il, so that the components of the signal which represent black will appear as black in the reproduced image and the light gradations from black to white represented by various videofrequency components will be properly represented in the reproduced scene, the diode 36 of the stabliizer l6 derives from the output of the amplifier l5 a positive unidirectional voltage equal to the peak value of the signal in the black direction. More particularly, since the load circuit of diode 36 efiectively comprises resistor 31 and condenser 35, diode 36 operates as a peak rectifier and passes current only during the negative peaks of the applied detected but unsta-- bilized signal which is of the wave form illustrated in Fig. 3 but centered about an equal area axis by the alternating current coupling condenser 35. The diode load circuit 35, 31 is so proportioned that there is thus built up across the resistor 3'! a bias voltage which Varies with the amplitude of these negative peaks of the unstabilized signal. This bias voltage is applied positively from the upper terminal of resistor 31 to the control element of the device ll. The control element of the device I! is initially negatively biased from a suitable source G, and this bias is preferably such as to correspond approximately to black, the resultant of the bias voltage developed across resistance 31 and the bias voltage G being utilized for stabilizing the signal input to unit ll at the blanking level. The resultant video-signal applied to the control element of the device ll, therefore, is effectively stabilized with respect to the blanking level of the signal; that is, to a level representing black or a value blacker than black having a fixed value with respect to black. The signal, therefore, as applied to the control element has a wave form such as is illustrated by the curve of Fig. 5. It will be noted, however, that this signal still includes the synchronizing pulses with reverse polarity which were inserted by means of the connection from the output circuit of the amplifier l9. Since these pulses are of such polarity that they would appear as white in the reproduced image, it is necessary that they be effectively removed from the signal. For this purpose, therefore, as pointed out above, the synchronizing pulses developed by the amplifier H] are applied negatively to a suitable control electrode of the cathode-ray tube I1 and, in case they are applied to the control grid of image-reproducing device H, the synchronizing-pulse components are reinserted in the black direction in the blanking-level-stabilized signal input to image-reproducing device ll. Hence, the resultant signal developed by the device ll has, in effect, a wave form such as is illustrated by the curve of Fig. 6 having a predetermined fixed black level and being reduced to zero during retrace periods. It will thus be seen that the present invention provides a television receiver which includes improved control means for effecting the setting of theblacklevel of the signal at a proper value for reproduction; as well as automatic amplificaw tion control. a v,

Whilethere has been described what is at present considered to be the: preferred embodiment of this invention, it will be obvious to, those'skilled in the art that various changes and: modifications may be made therein without departing from the invention, and it is, therefore, aimed inthe appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is: x

1. A television receiver adapted for the receptionof a television signal carrier the amplitude of which may vary at the. receiver and modulated during trace periods by light componentshaving a predetermined blanking level, unmodulatedcby light components and modulated by synchroniz ing-pulse components outside the amplitude range of said light-modulation components during retrace periods, comp-rising means for deriving from a received signal its modulationsignal stabilized with respect to the peaks of said synchronizing pulse components .'means for blocking out only said synchronizing-pulsecomponents irom said-- derived signal, means for stabilizing the resultant derivedlsignal with respect tosaid blanking level,

and means for utilizing the blankinglevel stabi lized signal to reproduce the transmitted light modulation components.

during trace periods by light components having a predetermined blanking level, unmodulated by light components and modulated by Sy Chi nizing-pulse components outside the amplitude range of said light-modulation components during retrace periods, comprising means for deriving from a received signal its modulation signal stabilized with respect to the peaks of said synchonizingpulse components, means for separately deriving from said received signal said synchronizing-pulse components, means for utilizing said separately derived synchronizing-pulse components for blocking out corresponding components from said derived modulation signal, means for stabilizing the resultant signal with respect to said blanking level, and means for utilizing the blanking-levelstabilized signal to reproduce the transmitted light modulation components.

3. A television receiver adapted for the reception of a television signal carrier the amplitude of which may vary at the receiver and modulated during trace periods by light components having a predetermined blanking level, unmodulated by light components and modulated by synchronizing-pulse components outside the amplitude range of said light-modulation components during retrace periods, comprising means for deriving from a received signal its modulation signal stabilized with respect to the peaks of said synchronizing puse components, means for blocking out only said synchronizing-pulse components from said derived signal, a diode rectifier for deriving from the resultant derived signal a positive bias voltage proportional to the peak value of said signal on the black side of its axis, means for utilizing said bias voltage for stabilizing said resultant derived signal with respect to said blanking level, and means for utilizing the blanking-level-stabilized signal to reproduce the transmitted light of which may vary at the .receivenand modulated duringtrace periods by light components. having a predetermined blanking level, unmodulated: by light components/and, modulated by synchronizing-pulse components: outside the, amplitude range of said light-modulation. components during re trace periods, comprising means for derivin'g froma received signal itsmodulation signal stabilized with respect. to. the peaks of said synchronizingpulse components, means for blocking out only saidisynchronizing-pulse components in said signal, a source of negative bias voltage oi a fixed value, a diode rectifier for deriving from the-resultant derived signal'a positive control-bias volt age proportional to the peak value of said signal on the-black side of its zero axis; means for flutilizing the resultant of said two bias voltages for stabilizing the resultantsignal wtih respect 'to' said blanking level, and means for utilizing the resultant signal to reproduce the transmitted light' modulation components; p

5. A television receiver adapted for thereception of a television signal carrier the amplitude of' black, -unmodulated by light: components and modulated by synchronizing pulse 'cornponentjs' outside the amplitide range of -said light-modular.

tion components during retrace periods; comprismg means for deriving from a received signal its modulation signal stabilized with respect to the peaks of saidsynchronizing pulse 7 components;

means for blocking out only said synchronizing .pulse components in said-signal, a source of neg-1' ative-bias voltage having a" fixed value with re spect to black, means for deriving-from theresulte" ant derived signal'a positive control-bias voltage, proportional to the peak' value thereof -on'the' black side of its zero axis, means utilizing the resultant of said two bias voltages to stabilize the resultant signal with respect to said blankin level, thereby to establish a fixed level for signal components corresponding to black, and means for utilizing the resultant signal to reproduce the transmitted light modulation components.

6. A television receiver adapted for the reception of a television signal carrier the amplitude of which may vary at the receiver and positively 7 modulated during trace periods by background and video-frequency light components, having a predetermined blanking level, unmodulated by light components and modulated by negatively poled synchronizing-pulse components outside the amplitude range of said light-modulation components during retrace periods, comprising inserting said synchronizing-pulse components in the black direction in the stabilized resultant signal, and means for utilizing the blanking-levelstabilized signal with reinserted synchronizingpulse components to reproduce the transmitted light modulated components.

7. A television receiver adapted for the recep- I tion of a television signal carrier the amplitude of which may vary at the receiver and positively V modulated during trace periods by background and video-frequency light components having a predetermined blanking level, unmodulated by Y light components and modulated by synchronizing-pulse components outside the amplitude range of said light-modulation components during retrace periods, comprising means for deriving from a received carrier signal its modulation signal stabilized with respect to the peaks of said synchronizing pulses, means for blocking out only said synchronizing-pulse components in said signal, means responsive to variations in the value of a predetermined level in the resultant modulation signal for developing a controlling effect, means for utilizing said controlling eifect to adjust an operating characteristic of the receiver, means for stabilizing said resultant derived signal with respect to said blanking level, and means for utilizing the blanking-level-stabilized signal to reproduce the transmitted light modulation components.

8. A television receiver adapted for the reception of a television signal carrier the amplitude of which may vary at the receiver and positively modulated during trace periods by background and video-frequency light components having a predetermined blanking level, unmodulated by light components and modulated by synchronizing-pulse components outside the amplitude range of said light-modulation components during retrace periods; comprising means for deriving from a received carrier signal its modulation signal stabilized with respect to the peaks of said synchronizing pulses, means for blocking out only said synchronizing-pulse components in said signal, means responsive to variations in the value of a predetermined level in the resultant modulation signal for developing a controlling Y effect, means for utilizing said controlling efl'ect to adjust an operating characteristic of the receiver, means for stabilizing said resultant derived signal with respect to said blanking level,

means for reinserting said synchronizing-pulse components in the blanking-level-stabilized signal, and means for utilizing the blanking-levelstabilized signal with reinserted synchronizingpulse components to reproduce the transmitted light modulation components.

9. A television receiver adapted for the reception of a television signal carrier the amplitude of which may vary at the receiver and positively modulated during trace periods by background and video-frequency light components having a predetermined blanking level, unmodulated by light components and modulated by synchronizing-pulse components outside the amplitude of said light-modulation components during retrace periods, means for deriving from a received carrier signal its modulation signal stabilized with respect to said synchronizing pulses, means for separately deriving from said received signal said synchronizing-pulse components, means for utilizing said separately derived synchronizingpulse components for blocking out only the corresponding components in said derived modulation signal, means responsive to the blanking level of said stabilized modulation: signal for developing a controlling effect, means for utilizing said controlling effect to adjust an operating char--.

HAROLD M. LEWIS. 

